IBM Selective Sequence Electronic Calculator: История и конструкция

IBM Selective Sequence Electronic Calculator

The IBM Selective Sequence Electronic Calculator (SSEC) was an electromechanical computer built by IBM. Its design was started in late 1944 and it operated from January 1948 to August 1952. It had many of the features of a stored program computer, and was the first operational machine able to treat its instructions as data, but it was not fully electronic. The President of IBM, Thomas J. Watson Sr., did not like Aiken's press release that gave no credit to IBM for its funding and engineering effort. Watson and Aiken decided to go their separate ways, and IBM began work on a project to build their own larger and more visible machine. Astronomer Wallace John Eckert of Columbia University provided specifications for the new machine; the project budget of almost $1 million was an immense amount for the time. Francis "Frank" E. Hamilton (1898–1972) supervised the construction of both the ASCC and the SSEC. Robert Rex Seeber Jr. was also hired away from the Harvard group, and became known as the chief architect of the new machine. Modules were manufactured in IBM's facility at Endicott, New York, under Director of Engineering John McPherson after the basic design was ready in December 1945.

The February 1946 announcement of the fully electronic ENIAC energized the project. The new machine, called the IBM Selective Sequence Electronic Calculator (SSEC), was ready to be installed by August 1947. It was dedicated and first demonstrated to the public on January 27, 1948. A. Wayne Brooke served as the chief electronic engineer for the machine's operation starting in 1950. Herb Grosch, the second person with a Ph. D. hired by IBM, was one of its first programmers. Another early programmer was Edgar "Ted" Codd. Elizabeth "Betsy" Stewart was chief operator, and often appeared in publicity photos. The SSEC was an unusual hybrid of vacuum tubes and electromechanical relays. Approximately 12,500 vacuum tubes were used in the arithmetic unit, control, and its eight (relatively high speed) registers, which had an access time of less than one millisecond. About 21,400 relays were used for control and 150 lower speed registers, with an access time of 20 milliseconds. The relay technology was similar to the ASCC, based on technology invented by Clair D. Lake (1888–1958). The arithmetic logic unit of the SSEC was a modified IBM 603 electronic multiplier, which had been designed by James W. Bryce. The bulky tubes were military surplus radar technology, which filled one entire wall. The memory was organized as signed 19 digit decimal numbers. Multiplication was computed with 14 digits in each factor. Most of the quoted 400,000 digit capacity was in the form of reels of punched paper tape. Addition took 285 microseconds and multiplication 20 milliseconds, making arithmetic operations much faster than the Harvard Mark I. Data that had to be retrieved quickly was held in electronic circuits; the remainder was stored in relays and as holes in three continuous card stock tapes that filled another wall. A chain hoist was needed to lift the heavy reels of paper into place. The machine read instructions or data from 30 paper tape readers connected to three punches, and another a table look up unit consisted of another 36 paper tape readers. A punched card reader was used to load data, and results were produced on punched cards or high speed printers. ENIAC co designer J. Presper Eckert (no relation to the IBM Eckert) called it "some big monstrosity over there that I don't think ever worked right". Seeber had carefully designed the SSEC to treat instructions as data, so they could be modified and stored under program control. IBM filed a patent based on the SSEC on January 19, 1949, which was later upheld as supporting the machine's stored program ability. Each instruction could take input from any source (electronic or mechanical registers or tape readers) store the result in any destination (electronic or mechanical registers, tape or card punch or printer), and gave the address of the next instruction, which could also be any source. This made it powerful in theory.

The first application of the SSEC was calculating the positions of the Moon and planets, known as an ephemeris. Each position of the Moon required about 11,000 additions, 9,000 multiplications, and 2,000 table look ups, which took the SSEC about seven minutes. This application used the machine for about six months; by then other users were lined up to keep the machine busy. that the JPL Ephemeris Tape System was "used for virtually all computations of spacecraft trajectories in the US space program", and that it had, as its current lunar ephemeris, an evaluation of the Improved Lunar Ephemeris incorporating a number of corrections: sources are named as 'The Improved Lunar Ephemeris' (documentation which was the report of the Eckert computations carried out by the SSEC, complete with lunar position results from 1952 to 1971), with corrections as described by Eckert et al. (1966), and in the Supplement to the AE 1968. Taken together, the corrections thus referenced modify practically every individual element of the lunar computations, and thus the space program appears to have been using lunar data generated by a modified and corrected derivative of the computational procedure pioneered using the SSEC, rather than the directly resulting tables themselves. The first paying customer of the SSEC was General Electric. The SSEC was also used for calculations by the U. S. Atomic Energy Commission for the ANP project to power an airplane with a nuclear reactor. Robert D. Richtmyer of Los Alamos National Laboratory used the SSEC for some of the first large scale applications of the Monte Carlo method. Llewellyn Thomas solved problems with stability of laminar flow, programmed by Donald A. Quarles Jr. and Phyllis K. Brown. In 1949, Cuthbert Hurd was hired (also after a visit to the SSEC) and started a department of applied science; the operation of SSEC was eventually put into that organization.

The SSEC room was one of the first computers to use a raised floor, so visitors would not see unsightly cables or trip over them. The large array of flashing lights and noisy electro mechanical relays made IBM very visible to the public. The SSEC appeared in the film Walk East on Beacon, which is based on a book by J. Edgar Hoover. The SSEC attracted both customers and new employees. Both Hurd and Backus were hired after seeing demonstrations of the facility. The 1946 ENIAC had more tubes than the SSEC and was faster in some operations, but was originally less flexible, needing to be rewired for each new problem. At the end of 1948 a new IBM 604 multiplier was announced, which used newer tube technology that already made the bulky tubes of the SSEC obsolete. By May 1949 the Card Programmed Electronic Calculator was announced, and shipped in September. It was effectively a much scaled down version of the SSEC technology to allow customers to perform similar calculations. In July 1953 the much less expensive (and even better selling) IBM 650 was announced, which had been developed by the same Endicott team who developed the SSEC.